Susceptibility

Q: What is magnetic susceptibility

A: When a substance is placed in a magnetic field it is the susceptibility of the substance to have its own field. When a ferrous nail is placed in a magnetic field, the nail is “ferromagnetic, resulting in a much higher magnetic field. Water is slightly diamagnetic, resulting in a lower field within the water. Gadolinium is paramagnetic creating a slightly stronger field. The implication for imaging is that the process of image acquisition/creation assumes that the magnetic field is constant. However, since the field changes at air/water or bone/marrow interfaces, there is potential for geometric distortion/signal loss.

Susceptibilty can be thought of as the ratio of the substance's magnetization to the applied field H 

Iron is about 200,000

https://en.wikipedia.org/wiki/Magnetic_susceptibility

Q: Order these sequences by their sensitivity to susceptibility effects: conventional spin echo, fast spin echo, gradient recalled echo.

A: fast spin echo, conventional spin echo, gradient recalled echo.

What is the advantage and disadvantage of the sensitivity of FSE T2 to susceptibility effects?

For the interested reader, a reference about spine instrumentation and susceptibility artifact:

http://pubs.rsna.org/doi/abs/10.1148/radiology.204.1.9205258